The shooting occurred at about 3:30 pm, after school at Wendell Phillips College High School, Chicago Police Chief Detective Brian Dinihan said at a news conference.
The guard opened the door of the school. “Immediately, a criminal on the street started shooting in the direction of the guard,” Dinihan said.
Deenihan said that the guard had been shot several times and the bullet passed through an unopened door and hit the teenager. She was taken to the hospital in critical condition on Tuesday.
According to the police, the 45-year-old security guard was also taken to the hospital in good condition.
Investigators did not know whether the shooting was targeted. The police added that no one was arrested.
The shooting took place over the weekend and dozens of people were shot dead in the city.
According to the major incident notice from the Chicago Police Department, 31 shootings occurred in Chicago during the holiday weekend, and 40 people were shot dead. Four victims were killed.
Carma Hassan of CNN contributed to this story.